Epidemiologic characterizations of the prevalence of Chronic Fatigue Syndrome are derived largely from data collected in treated populations, and these findings might be biased by differential access to health care treatment by gender, racial/ethnic and social class status. The goal of this study is to do a community-based prevalence study. the specific aims are: 1) to determine the rate of CFS in a socioeconomically and ethnically diverse sample 26,000 adults in Chicago; 2) to establish the relative prevalence of CFS across race/ethnicity, socioeconomic status and gender; and 3) to examine comorbidity between CFS and psychiatric disorders. It is hypothesized that the prevalence of CFS is higher than what has been found from clinically-based estimates. This study will be carried out in three stages. First, there will be an initial screening of a Chicago area sample. Respondents who meet CFS screening criteria will be followed up with a detailed structured psychiatric assessment. Stage three will involve a detailed medical history, physical exam, and laboratory tests. Univariate and multivariate statistical techniques will be utilized to delineate the overall rate of CFS in this Chicago population, its relative prevalence by gender, race/ethnicity, and social class, the prevalence of psychiatric comorbidity, and levels of functional impairment. Different definitions of CFS will be employed, and they will be compared and contrasted.